1. Field of the Invention
This invention relates to a method of minimizing image signal noise, and an apparatus for carrying out the method. This invention also relates to a radiation image read-out method for reading out an image stored on a stimulable phosphor sheet by exposing the stimulable phosphor sheet to stimulating rays which cause the stimulable phosphor sheet to emit light in proportion to the stored radiation energy, and photoelectrically detecting the emitted light, and an apparatus for carrying out the radiation image read-out method.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor in a radiation image recording and reproducing system. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to a radiation passing through an object such as the human body to have a radiation image of the object stored thereon, and is then exposed to stimulating rays which cause the stimulable phosphor sheet to emit light in proportion to the stored radiation energy. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted to an electric image signal by a photodector, and the radiation image of the object is reproduced as a visible image by use of the image signal on a recording medium such as a photographic film, a display device such as a cathode ray tube (CRT), or the like.
The radiation image recording and reproducing system using a stimulable phosphor sheet is advantageous over conventional radiography using a silver halide photographic material in that the image can be recorded over a very wide range (latitude) of radiation exposure. More specifically, since the amount of light emitted upon stimulation after the radiation energy is stored on the stimulable phosphor sheet varies over a wide range in proportion to the amount of said stored energy, it is possible to obtain an image having desirable density regardless of the amount of exposure of the stimulable phosphor sheet to the radiation, by reading out the emitted light with an appropriate read-out gain and converting it into an electric signal to reproduce a visible image on a recording medium or a display device.
Therefore, in the aforesaid radiation image recording and reproducing system, high sensitivity image recording may be conducted by adjusting the radiation dose to a very low value. In the case of general radiation image recording, the amount of exposure to the radiation varies locally on a single stimulable phosphor sheet in accordance with differences in radiation absorptivity among various portions of an object. When the radiation dose is adjusted to be very low as mentioned above, the radiation energy stored on the portion of the stimulable phosphor sheet where the amount of exposure to the radiation is comparatively low becomes markedly low. The amount of light emitted by such a portion of the stimulable phosphor sheet naturally becomes of a markedly low level. In the aforesaid radiation image recording and reproducing system, even light of such a low level can be read out. However, the level of a noise component becomes comparatively high in the image signal obtained by detecting the light of such a low level, and graininess at the portion represented by the image signal is deteriorated in the reproduced visible image.
The problem of deterioration of graininess may be solved by processing the image signal, for example, by passing the image signal through a low-pass filter and eliminating the noise component in the high-frequency region. However, with this method, also the high-frequency component of the image signal representing the detail component of the image is decreased over the overall region, and sharpness of the reproduced visible image becomes low also at the image portion where the amount of exposure to the radiation is comparatively high. In general, the image portion where the amount of exposure to the radiation is comparatively high is the portion at which the detail component is to be observed in detail. Therefore, the reproduced visible image having sharpness deteriorated at said portion becomes unsuitable for viewing, particularly for diagnostic purposes.